[hobgoblin351]

A highrise manifold, either a single plain or a duel will usually be more efficient than a stock one. It will allow you to flow more air thus increasing you HP output. In the case of say a single plain highrise it will allow you to raise the RPM range of the engine and run bigger cams. How much depends on what size cam, and other changes made to the engine. If all you are changeing is the manifold you'll probably only add around 15-20 horses.
As far as clearence I couldn't tell you, it depends on the manifold/carb/aircleaner setup. Good luck